This invention relates to machines for manufacturing corrugated board, and in particular to an improvement wherein an air bar, having a length that can be varied to match the width of the web to which the glue is being applied, is employed to maintain proper pressure holding the web against the glue applicator roll.
Conventionally, corrugating machines begin with a liner, that is, a single sheet of paper. To this liner the machine applies a fluted medium, by use of glue, to form a web. Then a second liner is glued to the flute tips of the web opposite the first liner to complete the process. In the process of gluing this second liner to the web, conventional corrugating machines apply glue to the flute tips by use of an applicator roll and a rider roll. The applicator roll turns at or just below the speed of the web, while its lower portion is immersed in a tank of glue. As it turns, of course, some of the glue adheres to the roll surface as it leaves the tank. When the surface of the applicator roll then touches the flute tips of the web, some of the glue is applied to the flute tips. In order to ensure that each flute tip does touch the applicator roll so as to have glue applied to it, pressure must be applied to the back of the first liner. In a conventional corrugating machine, this pressure behind the first liner is applied by a rider roll made of metal or other hard material. Such a rider roll must be very carefully controlled, however, since the flute tips can be crushed if the gap between the rider roll and the applicator roll becomes too small, or the flute tips could lift off the applicator roll and receive no glue if the gap becomes too large.
Even if the rider roll is sufficiently controlled so that crushing and liftoff are avoided, when the pressure exerted by the rider roll is only slightly too great, too much glue can be applied to the flute tip. When too much glue is thus applied, the result can be "washboarding." Washboarding is a physical fault in corrugated board wherein the liner, instead of forming a flat surface, partly follows the contours of the fluted medium to produce generally parallel alternate ridges and valleys. It is clear that printing on such an uneven surface is difficult. The unevenness makes it necessary for the printer to increase the depth of the die impression, which can again crush the board to some extent. In order to avoid this effect, control mechanisms have been designed, often at great expense, to exactly control the position and pressure of the rider roll.
This invention relates to solutions to the problems described above.